Oxidized cellulose has been known in the art for more than 50 years. Its ability to be absorbed by the body makes oxidized cellulose an attractive material for medical uses such as sutures, hemostats, wound coverings, and adhesion prevention devices. Several oxidizing agents can be used to oxidize cellulose including metaperiodate, hypochlorite and dichromate. However, the only suitable method for preparing material that is bioabsorbable and maintains the appropriate physical properties is by oxidation with nitrogen dioxide.
Early work on the oxidation of cellulose to produce bioabsorbable cellulose was conducted by W. O. Kenyon and others at Tennessee Eastman (for example see U.S. Pat. No. 2,423,707 issued Jul. 8, 1947 to Kenyon et al., and R. H. Hasek et al. “Oxidation of Cellulose,” Ind. & Eng. Chem., Vol. 41, p. 2 (1949)). In these pioneering oxidation processes, they found that cellulose was oxidized by using either gaseous nitrogen dioxide or a solution of nitrogen dioxide in a chlorinated hydrocarbon solvent such as carbon tetrachloride.
Carbon tetrachloride was initially investigated by Kenyon as an inert solvent for oxidation of cellulose with nitrogen dioxide. Other solvents were also tested by scientists in the field. W. H. Ashton et al. in U.S. Pat. No. 3,364,200 discloses a process for preparing oxidized cellulose in nonaqueous solvents such as Freon 113 (CCl2FCClF2) and Freon 11 (CCl3F). The use of chlorinated hydrocarbons and chlorinated fluorocarbons (CFCs) were disclosed by B. G. Yasnitsky in U.S. Pat. No. 4,347,057.
The success of these solvents turned on their failure to interact with the oxidizing agent. It is critical for the oxidation process that the solvent not react with the oxidizing agent (nitrogen dioxide) otherwise the solvent will be destroyed and the oxidizing agent will be spent in reacting with the solvent rather than with the cellulose material. Organic solvents containing a carbon hydrogen bond are all susceptible to reaction with nitrogen dioxide. This need for the solvent to be inert with respect to nitrogen dioxide severely limits the number of organic solvents available for use in this reaction.
Further, there were problems associated with the inert organic solvents used in this process. Chlorinated hydrocarbons and chlorinated fluorocarbons (Freon type solvents) were found to pose environmental problems related to the depletion of the ozone layer at high altitudes. It is believed that the carbon chlorine bond (C—Cl) in these molecules is ruptured in the stratosphere to produce chlorine radicals which go on to destroy ozone. Carbon fluorine bonds (C—F) are stronger than carbon chlorine bonds and are not ruptured by radiation in the stratosphere, thus fluorocarbons do not deplete the ozone layer.
In an effort to oxidize cellulose and minimize the problems associated with the organic solvents, U.S. Pat. No. 5,180,398 by Boardman et al. teaches a process for oxidizing cellulose with a solution of nitrogen dioxide in a perfluorohydrocarbon solvent. The perfluorohydrocarbon solvents that are used are inert to the nitrogen dioxide and supposedly do not destroy the ozone layer of the earth. U.S. Pat. No. 5,914,003 by Kosowski et al. also discloses a process for effectively oxidizing cellulose with nitrogen dioxide but uses a solution of a hydrofluoroether solvent. However, the hydrofluoroether solvents cited by the author are not fully fluorinated compounds but, in fact, contain carbon hydrogen bonds and thus are not totally inert to the oxidizing agent.